Many different types of vehicle detection systems are presently in widespread use, either to control traffic signals, or for other purposes. One such prior art detection system uses an electric switch which is mechanically operated by a treadle mounted on the surface of the roadway. However, such mechanical treadle switches are expensive to install. Moreover, they are subject to considerable wear, and they require frequent maintenance.
Sub-surface magnetic inductance vehicle detection systems are also in use which employ inductive wire loops buried beneath the surface of the roadway, and whose inductance changes as a vehicle crosses into the area circumscribed by the loop. Such prior art detection systems usually include an electronic detector circuit which senses the changes in inductance in the loop by measuring a change in frequency of the output of an oscillator, thereby detecting the presence of the vehicle.
Such a magnetic inductance vehicle detection system is described, for example, in U.S. Pat. No. 3,875,555 which is assigned to the present assignee. The system described in the patent is particularly suited for application in operating traffic actuated traffic lights. The system of the patent includes a reference oscillator, and a loop oscillator which exhibits a frequency differential with the reference oscillator in the presence of a vehicle over the loop which forms the resonant circuit for the loop oscillator. This frequency differential is sensed by the system, and if it exceeds a particular threshold, an output call signal is produced. The system described in the patent also incorporates an automatic tracking circuit between the two oscillators so that the loop and circuit changes due to environmental and other conditions are automatically compensated. The frequency differential is detected by counting the successive cycles of the output signals of the oscillators over a predetermined time interval.
The reference oscillator is normally tuned to the same frequency as the loop oscillator, and the two oscillator output signals are formed into square waves which are divided down by means of digital counters. Whenever the frequency of the loop oscillator is increased due to the presence of a vehicle until the frequency differential between it and the reference oscillator exceeds a particular threshold, a square wave appears in the system which is converted into a direct current voltage call signal. The call signal is used in appropriate control circuitry to activate the traffic lights. Different sensitivities may be selected by changing the division factor of the digital counters.
As mentioned above, the system of the patent includes an inductive loop embedded in the roadway and which is used as part of the resonant circuit of the loop oscillator. This means that the frequency of the loop oscillator is a function of loop inductance, and whenever a vehicle enters the area circumscribed by the loop, the inductance of the loop drops; which, in turn, increases the frequency of the loop oscillator. It is clear, therefore, that should the loop oscillator frequency decrease with respect to the reference oscillator frequency, such a decrease is due only to environmental drift conditions, so that the tracking circuit may compensate the frequency differential on a rapid basis.
However, when the frequency of the loop oscillator increases with respect to the reference oscillator, the increase may be due to the presence of a vehicle, or to environmental drift conditions. Therefore, the compensation by the tracking circuit for the later instance must proceed in a slower basis than in the former instance, so that compensation is provided only in the presence of slow environmental drifts, rather than in response to a sharp increase in frequency due to the presence of a vehicle. To achieve stability, the loop and reference oscillators must track in terms of frequency. The frequency difference between the two oscillators is monitored in the system described in the patent, so that whenever the loop oscillator frequency becomes lower than the reference oscillator frequency, indicating an environmental frequency drift condition, an error signal is produced. This error signal is in the form of a pulsating signal, and it is fed to the input of a digital integrator. The resulting output of the integrator is used to increase the loop oscillator frequency at a relatively high correction rate, and this continues until the error signal disappears.
On the other hand, whenever the frequency of the loop oscillator becomes higher than the frequency of the reference oscillator in the system described in the patent, a condition which may be caused either by variations in the loop inductance due to environmental changes, or to the presence of a vehicle over the loop, the tracking circuit develops a low rate error signal which is introduced to the input of the digital integrator so that correction to reduce the frequency of the loop oscillator proceeds at a relatively low rate for long term automatic tracking to compensate for slow environmental drift conditions, but which does not respond to rapid frequency changes due to the presence of a vehicle.
The microprocessor system of the present invention provides the same functional operations as the system described in the patent. However, in the system of the invention a single loop oscillator is used in conjunction with a number of separate loops, which are located, for example, at each corner of an intersection, and the microprocessor serves sequentially to connect each loop to the common oscillator on a multiplexed time-shared basis. All of the detector functions performed by the circuitry described in the patent, as discussed above, are performed by the microprocessor.